Transcutaneous power optimization circuit for a medical implant

ABSTRACT

In a cochlear implant system, the implantable stimulator includes a monitor which monitors parameters associated with the stimulation signals and/or the power stored in an energy storage element which stores energy transmitted from the processor. This parameter or parameters is/are analyzed and one or more feedback signals are generated and transmitted back to the processor. The processor uses the feedback signal to insure that power is transmitted to the stimulator optimally and that the stimulation signals are compliant.

BACKGROUND OF THE INVENTION

A. Field of Invention

This invention pertains to an optimization circuit in a cochlear implantsystem and more particularly to a circuit which monitors one or moreparameters within the implant such as the internal power supply leveland the compliance of the stimulation signals applied by the implant. Ifan undesirable condition is indicated by these parameters, the circuitgenerates control signals to correct the condition by adjusting thecoupling between the internal and external components of the system.

B. Description of the Prior Art

Certain patients suffer from a hearing disability in the inner ear whichcannot be satisfactorily assisted by normal hearing aids. However, ifthe aural nerve is intact, the patient may have some aural functionsrestored with a cochlear implant system. A typical cochlear implantsystem presently available includes an external component or processorand an internal component often called the implanted stimulator. Theexternal component includes a microphone for receiving ambient soundsand converting them into electrical signals, a processor for processingsaid electrical signals into encoded signals and a transmittertransmitting said encoded signals to the internal component.

The internal component includes a receiver receiving the encodedsignals, a decoder for decoding said signals into stimulation signalsand an electrode array including both intracochlear electrodes extendinginto the patient's cochlear and optionally one or more extra-cochlearelectrodes. The stimulation signals are applied in the form of pulseshaving durations and waveshapes determined by the processor.

Because the internal component of the cochlear implant system isrelatively small, it is not normally provided with its own permanentpower supply. Instead, the internal component is energizedtranscutaneously by RF signals received from the external component withthe use of two inductively coupled coils, one provided in the externalcomponent and the other being provided within the internal component.The external component sends data to the internal component, by firstencoding the data into the RF signals and then transmitting it acrossthe transcutaneous link. The internal component decodes the data fromthe received RF signals and also stores the received RF energy in acapacitor to power its electronics. In order to achieve efficient powertransfer across the transcutaneous link, both coils are tuned toresonate, at or close to the operating frequency of the transmitter andare held in axial alignment with the aid of a magnetic coupling.

The amount of energy being transferred to the internal component dependsmainly on the amount of inductive coupling between the two coils as wellas the resonance frequency of the respective coils. The former isdependent on the thickness of the tissue separating the two coils, whichthickness varies over the patient population. Hence, for identicalcochlear implant systems the efficiency of energy transfer varies fromone patient to another.

The required amount of energy varies with the patient, (due to theelectrode-tissue interface impedance being patient specific) the systemprogramming, and the sound environment. Therefore, every cochlearimplant system must be designed so that adequate power is delivered tothe internal component for all patients under all conditions. Hence,there is an excess energy transfer across the link for patients withrelatively smaller separation between the coils, or a lowelectrode-tissue interface impedance, resulting in a shorter batterylife, than optimally desired.

Attempts have been made by others to resolve this problem but they havenot been entirely satisfactory. For example, U.S. Patent No. 5,603,726discloses a multichannel cochlear implant system in which theimplantable section generates signals to a wearable processor indicativeof the status of the implantable section, such as its power level andstimulation voltages. The information is used by the wearable processorto modify the characteristics of the signals transmitted. Moreparticularly, the implantable section has an internal power supplycapable of producing several outputs having different nominal DC levels.Additionally, the implantable section is also capable of providingunipolar or bipolar stimulation pulses between various intercochlearelectrodes as well as an indifferent electrode. A telemetry transmitteris used to send data to the wearable processor, the data beingindicative of the voltage levels of the power supply outputs, theamplitudes of the stimulation signals and other parameters. The wearableprocessor uses the power level signals to adjust the amplitude (andtherefore the power) of the RF signals transmitted to the implantablesection. However, this approach is disadvantageous because it requiresan RF transmitter having a variable programmable amplitude, and utilizesa fixed tuning of the transmit coil, therefore making no attempt tomodulate the voltage on the tank capacitors to track the voltagerequired to maintain system compliance. Obviously such a transmitter isexpensive to make and more complex then a standard RF transmitter havinga preset amplitude. Moreover, sending information from the implantablesection about the amplitude of the stimulation pulses after these pulseshave already been applied is ineffective because, if one of these pulsesis out of compliance, the external section can do nothing about it,except crank up the power to insure that future pulses are compliant.However, merely cranking the power without any further intelligencewastes energy.

Commonly assigned application Ser. No. 09/244,345 filed Feb. 4, 1999entitled HIGH COMPLIANCE OUTPUT STAGE FOR A TISSUE STIMULATOR,incorporated herein by reference, describes a cochlear implant systemwherein the generation of stimulation pulses is monitored, (i.e. thecompliance of the stimulation generation circuit) and a voltagemultiplier is used if necessary to ensure that the stimulation pulsesare of the desired intensity. This application essentially deals with asystem of improving the internal power supply in order to eliminatestimulation pulses, and as such, there is no provision in thisapplication for transmission of data back to the external section.

OBJECTIVES AND SUMMARY OF THE INVENTION

In view of the above disadvantages of the prior art, it is an objectiveof the present invention to provide a power control circuit for acochlear implant which is constructed and arranged to automatically anddynamically optimize the power transferred to the internal componentbased on one or more preselected criteria by adjusting an inductivecoupling therebetween.

A further objective is to provide a power control circuit for a cochlearimplant which is constructed and arranged to automatically anddynamically regulate the inductive coupling with the internal componentthereof to insure that power is not wasted, thereby increasing the lifeof the external component battery.

A further objective is to provide a cochlear implant system wherein theexternal and internal systems are coupled inductively, wherein thevoltage of the internal supply is monitored and the frequency of thiscoupling is tuned to obtain optimal power transfer using the voltage asa feedback signal.

Yet another objective is to provide a cochlear implant system whereinthe compliance of the stimulation signals is monitored and used as afeedback signal to optimize the power transfer to the internalcomponent.

Yet a further objective is to provide a cochlear implant with acompliance monitor arranged and constructed to sense a possible out ofcompliance condition before the respective stimulation pulse iscompleted and to adjust the power transferred to the internal section insuch a manner that the out of compliance condition is averted.

Other objectives and advantages of the invention shall become apparentfrom the following description.

Briefly, a cochlear implant system constructed in accordance with thisinvention includes an external speech processor and an implantablestimulator having electronic circuitry, the two components being coupledto each other inductively by respective coils. Each coil is part of atank circuit. The external speech processor transmits RF signals throughthe coupling. The implantable stimulator uses these signals for twopurposes. First, the energy of the signals is stored in a storageelement such as a capacitor and used to power the electronic circuitry.Second, the signals are decoded and used to derive the stimulationsignals applied to the aural nerve.

In one embodiment of the invention, a parameter indicative of thevoltage of the storage element is monitored and sent back to the speechprocessor via a secondary channel. The external speech processor thenadjusts the frequency of its tank circuit to regulate the powertransferred to the internal component to optimize it.

Additionally, or alternatively, the compliance of the stimulationsignals is monitored and used as a feedback signal to control thefrequency of the tank circuit to optimize power transfer to the internalcomponent. This adjustment can be done either based on statisticalbasis, or in response to an individual and specific out of compliancecondition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a cochlear system constructed inaccordance with the present invention;

FIG. 2 shows a schematic diagram of the external component of thecochlear system of FIG. 1;

FIG. 3 shows a schematic diagram of the internal component of thecochlear system of FIG. 1;

FIGS. 4A, 4B and 4C show the power control signals transmitted from theinternal to the external components respectively to indicate the powerlevel induced within the internal component;

FIG. 5A and 5B show flow charts for the operation of internal andexternal components of FIGS. 1-3, respectively; and

FIG. 6 shows two sets of typical biphasic stimulation signals defined bythe speech processor;

FIG. 7 shows the current pulses required to produce the stimulationpulses of FIG. 6; and

FIG. 8 shows the corresponding waveforms across the current source.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, a cochlear implant system 10 constructed inaccordance with this invention includes an external component 12 and aninternal component 14. The external component includes a speechprocessor 12A and is associated with a microphone 16 for sensing ambientsounds and generating corresponding electrical signals. These signalsare sent to the speech processor 12A which processes the signals andgenerates corresponding encoded signals. The encoded signals areprovided to a transmitter (including a transmit coil 20) fortransmission to the internal component 14.

The internal component 14 (which may also be referred to as animplantable stimulator) receives the power and data via a receive coil22. The RF power signal is stored by a power supply 24 (See FIG. 3)which provides power for the internal component 14. The data signalscontrol the operation of the internal component 14 so as to generate therequired stimulation pulses which are applied to the auditory nerve ofthe patient via an electrode array 28.

The structure of the external speech processor 12A is shown in moredetail in FIG. 2. First, the audio signals received from microphone 16are fed to a signal processor 30. This signal processor 30 maps theaudio signals into excitation signals in accordance with one or moremapping algorithms stored in a map memory 31. These excitation signalsare encoded by a digital data encoder 34. The encoder data is combinedwith an RF signal in the data and power transmitter 36, and passed tothe transmit coil 20 via a tuneable tank circuit 38.

In accordance with the present invention, encoded telemetry data isreceived back from the internal component 14 via coil 20, and is decodedby telemetry decoder 52. The decoder telemetry data is passed to thetuning adjuster and power controller 40, which uses the telemetry datato generate a tuning adjustment signal. The tuneable tank circuit 38adjusts the tuning of the transmit coil 20 according to the tuningadjustment signal as described in more detail below. This can beachieved, for example, by using an electrically controlled variablecapacitor in conjunction with a series tuning capacitor, or by any ofvarious similar means known to the art. Power to the whole system 10 isprovided by a power supply 50 which typically includes a battery.

Referring now to FIG. 3, the internal component 14 includes a housing(not shown) which is hermetically sealed. The component 14 also includesa receiver tank circuit 32 having the receive coil 22 and a capacitor66. Signals received through this tank circuit are fed to a power supply24 generating an output voltage Vdd. The power supply is represented inFIG. 3 by a diode 68 charging a capacitor 70. The power supply 24 usesthe energy of the received RF signals to charge up the capacitor 70.

The RF signals are also fed to a data decoder 60. The data decoder 60derives from the RF signal the digital excitation signals generated bythe data encoder 34 and generates corresponding stimulation controlsignals. These signals are fed to a programmable current source 62 and aswitching control circuit 64. These two circuits cooperate in responseto the signals from data decoder 60 to apply the cochlear stimulationsignals to predetermined electrodes of electrode array 28 in a knownmanner which is beyond the scope of this invention.

Implant 14 further includes a compliance monitor 66 which generates anoutput that is fed to a telemetry encoder 80 as discussed more fullybelow; and a power supply monitor 82 which is used to monitor thevoltage Vdd generated by power supply 24 and which provides a voltagecondition signal to telemetry encoder 80.

The compliance monitor 66 and power supply monitor 82 each sense certainspecific functions of the internal component and transmit them to thetelemetry encoder 80. The telemetry encoder 80 then transmits thisinformation to the telemetry decoder 52. The data is decoded and used toadjust the power transmit between the coils, if necessary.

An exemplary mode of operation indicating the voltage monitoring made isnow described in conjunction with FIGS. 4A, B and C and 5A and 5B. Atpredetermined intervals, for example, every 100 ms, or alternativelyafter every stimulation pulse, the telemetry encoder 80 generates afirst pulse F. (Step 100). This pulse may have a duration of about 1 ms.This pulse F indicates to the external speech processor 12A that theimplantable stimulator 14 is sending data.

Next, the power supply monitor 82 compares the power supply outputvoltage Vdd to a threshold value Vt and sends the result to thetelemetry encoder 80. More specifically, starting with step 102, thepower supply monitor 82 first determines if Vdd>Vt. If it is, then instep 104, a parameter pw (pulse width) is set to a predetermined valueA, of for example, 2 ms by the telemetry encoder 80.

If in step 102 Vdd is not larger than Vt then in step 106 a check isperformed to determine if Vdd is approximately equal to Vt. If it is,then in step 108 parameter pw is set to zero. If it is not then, Vddmust be smaller than Vt and in step 110 the parameter pw is set to apredetermined value B of, for example, 1 ms.

Next, in step 112 a pulse D is generated having a pulse width A or B, orno pulse is generated, depending on the outcome of the decisions 102 and106. The pulse D (if present) is generated a period T after pulse F. Tmay be about 1 ms. The results of this step are seen in FIGS. 4A, 4B,4C.

For FIG. 4A it has been determined that Vdd>Vt, and hence pulse D with apulse width A is sent about 1 ms after pulse F.

In FIG. 4B, Vdd has been found to be about equal to Vt and hence nopulse D is present.

In FIG. 4C, Vdd is found to be smaller that Vt and hence pulse D havinga pulse width B is sent about 1 ms after period F, pulse width B beinggenerally shorter than pulse width A. For example, pulse width A may be2 ms and pulse width B may be about 1 ms.

Pulse F and, if present, pulse D are then sent to the tank circuit 32.As a result, a corresponding signal appears on the transmit coil 20,which is then decoded by the telemetry decoder 52.

The operation of the telemetry decoder 52 is now described inconjunction with FIG. 5B. Starting with step 120, a pulse F is firstdetected which indicates that the power supply monitor 82 is sendinginformation about the status of the power supply 24. Next in step 122 acheck is made to determine if a pulse D is present following pulse F. Ifthis pulse is not detected, then in step 130 the previous operations arecontinued with no change.

If in step 122, a pulse D is detected then in step 124 a determinationis made as to whether this pulse D has a pulse width A or a pulse widthB. A telemetry pulse D having a relatively long pulse width, in a rangecorresponding to the pulse width A (for example if pulse D exceeds 1.5ms), indicates that the implant supply voltage is high (i.e. Vdd>Vt). Instep 126, the tuning adjuster and power controller 40 therefore adjuststhe tunable tank circuit 38 to reduce the power transferred to theimplant. A preferred method to accomplish this effect is to reduce theresonance frequency of the tank circuit.

If the telemetry pulse is less than 1.5 ms, (indicating a pulse width Band that the power supply Vdd<Vt) then in step 128 the tuning adjusterand power controller 40 adjusts the tunable tank circuit 38 to increasethe transferred power.

The tunable tank circuit 38 is adjusted by the tuning adjuster and powercontroller 40 via means of a tuning capacitor (not shown) which ispreferably a voltage dependent capacitor. It should be appreciated thatthe tunable tank circuit 38 could also be tuned by other known means aswould be understood by one skilled in the art.

Similarly, the above mentioned operation may be performed in respect ofthe compliance monitor signal, as described in more detail below.

Briefly, referring to FIG. 3; under the control of commands from datadecoder 60, the programmable current source 62 generates current pulseswhich are applied to the electrodes by switching control circuit 64.FIG. 6 depicts two typical stimulation current waveforms 70 and 73 whichmay be requested by the signal processor 30. It can be seen thateach-waveform, is biphasic, consisting of two current pulses of equalamplitude and opposite polarity. Thus, lower amplitude biphasic currentwaveform 70 consists of positive and negative pulses 71 and 72respectively, and higher amplitude current waveform 73 consists ofpositive and negative pulses 74 and 75.

Next, FIG. 7 depicts the corresponding current waveforms that must begenerated by the programmable current source 62 to produce the desiredstimulation current waveforms 70 and 73. That is, the programmablecurrent source 62 must generate two lower amplitude square waves 76 and77 to generate stimulus pulses 71 and 72 respectively, and two largeramplitude square waves 78 and 79 to generate the stimulus pulses 74 and75. Pulses 77 and 79 are reversed by the switching control circuit 64.However, if the current pulses 78 and 79 exceed the capability of thepower supply 24, an out of compliance condition occurs. This problem isresolved in the present invention as follows.

Referring to FIG. 8 the voltage waveform 80 represents the voltage Vn atthe output of the programmable current source 62. It can be seen fromthe shape of the voltage waveform 80 that the load contains a capacitivecomponent. The level Vc marks the minimum voltage across theprogrammable current source 62 at which compliance with the desiredcurrent waveforms of FIG. 7 can be maintained. The voltage Vca is alittle higher than Vc as shown and is selected to provide a safetymargin. As seen in FIG. 8, pulse 83 required to generate pulses 78 and74 of FIGS. 7 and 6 respectively, starts off at a level above Vca butdecreases linearly toward a minimum value (P) which is substantiallybelow level Vc and therefore is not attainable. When this pulse reachesVca (at point 85), the compliance monitor 66 generates a compliancemonitor signal indicating an out of compliance condition. The signal isencoded by the telemetry encoder 80 and transmitted to the externalprocessor. The signal may be the same signal as when VDD drops below VTas discussed above, or it may be a different signal, as would beappreciated by one skilled in the art. In response, the tuning adjusterand power controller commands the tunable tank circuit 38 to increasethe voltage transmitted to the internal section.

The adjustment of the link tuning or RF power generated can be performedfor every instance of a compliance monitor signal being received fromthe implant and may be maintained at a high level for a predeterminedtime, after which the RF power can be dropped to a previous level.

Alternatively, the frequency of the compliance monitor signal may bemonitored by the tuning adjuster and power controller 40. The linktuning or RF power generated could then be adjusted to maintain adesired ratio of compliance monitor signals to stimulation signals. Forexample, the link tuning or RF power generated could be adjusted to keepthe ratio of compliance monitor signals to stimulation pulses to adesired target of for example 5%, i.e. For this purpose, the tuningadjuster and power controller 40 includes a counter which counts everyinstance of non-compliance. After a predetermined number of stimulationpulses, for example a thousand, the counter is checked to determine thenumber of non-compliant instances. If the counter shows a number overthe desired target (i.e. 50 for a 5% target) then the tuning adjusterand power controller 40 adjusts the tank circuit 38 to increase itspower level. On the other hand for a number of non-compliant instancesbelow the target, the power level is increased. Of course, thisdetermination could also be made within the implant by the compliancemonitor itself, as would be evident to one skilled in the art.

Obviously numerous modifications can be made to the invention withoutdeparting from its scope as defined in the appended claims.

1. A cochlear implant system comprising: a speech processor including amicrophone receiving ambient sounds and converting them into soundsignals, a signal processor receiving said sound signals and convertingthem into stimulation signals, a transmitter for transmitting saidstimulation signals and including a tank circuit having a frequency ofresonance, an adjusting circuit arranged to control said frequency ofresonance in response to a feedback signal; and an implantablestimulator including a receiver for receiving said stimulation signals,an electrode for applying said stimulation signals to an aural nerve, anenergy storage element storing energy from said stimulation signals andproviding said stored energy to said stimulator, and a condition monitorthat monitors elements of said implantable stimulator and generates saidfeedback signal.
 2. The system of claim 1 wherein said stimulatorincludes an encoder for encoding said feedback signal, and wherein saidspeech processor includes a decoder for decoding said feedback signal.3. The system of claim 1 wherein said tank circuit includes a transmitcoil and tuning capacitance, said capacitance being adjustable by saidadjusting circuit.
 4. The system of claim 1 wherein said transmitter andreceiver are inductively coupled and wherein said feedback signal istransmitted from said stimulator to said speech processor through saidinductive coupling.
 5. The system of claim 1 wherein said conditionmonitor includes a compliance monitor that monitors said stimulationsignals as applied through said electrode, said compliance monitorgenerating said feedback signal in the form of a noncompliance signalindicative of when said stimulation signals are not in compliance. 6.The system of claim 1 wherein said condition monitor comprises a powersupply monitor that monitors an output of said power supply andgenerates said feedback signal to indicate a level of said power supplyoutput.
 7. A cochlear implant system comprising: a processor forconverting ambient sounds into stimulation signals, a transmitter fortransmitting said stimulation signals at a variable power leveldetermined by a feedback signal; and an implantable stimulator includinga receiver for receiving said stimulation signals, an electrode forapplying said stimulation signals to an aural nerve, a sensor thatsenses a parameter associated with said stimulation signals as they areapplied and a monitor that monitors said parameter and in responsegenerates said feedback signal.
 8. The system of claim 7 furthercomprising a power controller and wherein said transmitter generates RFsignals having an amplitude, frequency and duty-cycle, wherein saidpower controller adjusts said RF signals generated by said transmitterby changing one of said frequency, amplitude and duty cycle, in responseto said feedback signal.
 9. The system of claim 7 wherein said sensorsenses an amplitude of said stimulation signals and said monitorgenerates said feedback signal based on a range of said amplitude. 10.The system of claim 7 wherein said stimulator includes an encoder forencoding said parameter to generate an encoded signal which istransmitted to said processor and said processor includes a decoder fordecoding said encoded parameter.
 11. The system of claim 10 wherein saidtransmitter and receiver are coupled by an inductive coupling andwherein said encoded signal is transmitted through said inductivecoupling.
 12. The system of claim 7 wherein said monitor comprises acompliance monitor that determines when said stimulation signals arecompliant.
 13. The system of claim 12 further comprising a controllerthat determines a ratio of compliant stimulation signals to total numberof stimulation signals.
 14. The system of claim 13 wherein saidcontroller adjusts said power level to maintain said ratio at apredetermined target.
 15. The system of claim 12 wherein said compliancemonitor generates said feedback signal when said stimulation signals arenot compliant.
 16. The system of claim 15 wherein said processorincludes a power controller adapted to change said variable power levelfor a predetermined time period following said feedback signal.
 17. Thesystem of claim 15 wherein said processor includes a power controlleradapted to change said variable power based on a ratio defined by atotal number of stimulation signals and number of feedback signalsrelated to said stimulation signals.
 18. A cochlear implant systemcomprising: a processor for converting ambient sounds into stimulationsignals and including, a transmitter for transmitting said stimulationsignals and including a tank circuit having a frequency of resonance andan adjusting circuit arranged to control said frequency of resonance inresponse to a feedback signal; and an implantable stimulator including areceiver for receiving said stimulation signals, an electrode forapplying said stimulation signals to an aural nerve, an energy storageelement which stores energy from said stimulation signals and providespower to said stimulator, and a power monitor for monitoring a powerparameter associated with said energy storage element and generatingsaid feedback signal indicative of said parameter.
 19. The system ofclaim 18 wherein said stimulator includes an encoder for encoding saidpower parameter to generate an encoded signal which is transmitted tosaid processor and said processor includes a decoder for decoding saidencoded parameter.
 20. The system of claim 19 wherein said transmitterand receiver are coupled by an inductive coupling and wherein saidencoded signal is transmitted through said inductive coupling.